Mini Project- Audio Enhancement

Mini Project – Audio Processing
Author: University of Hertfordshire
Date created :
Date revised : 2009
Abstract
The following resources come from the 2009/10 BSc ( Hons ) in Music Technology (course number 2ELE0077) from the University of Hertfordshire. All the mini projects are designed as level two modules of the undergraduate programmes.
Project Objectives:
The objectives of this module are to demonstrate within the MATLAB® Simulink ® environment:
an understanding of the use of digital filtering for processing audio signals
the knowledge and ability to introduce echoes into digital speech/music
to develop a working knowledge of the mathematical theory introducing artificial reverberation into audio signals.
An important aspect of audio processing is to enhance the quality of audio (music) recording through the introduction of artificial reverberation.
The natural occurrence of reverberation in audio is due to a mixture of sound delays and reflections from different surfaces in the operating environment. When music is played in a closed environment (e.g. a hall), the sound reaches any given listener in the environment from various angles (with different levels of delay). Using digital signal processing, it is possible to artificially generate the effects of such multiple reflections of sound and thereby enhance the quality of a recorded audio. This is the basis of this project in which the students are required to develop systems for generating artificial echoes and reverberations for given audio files.
© University of Hertfordshire 2009. This work is licensed under a Creative Commons Attribution 2.0 License .

Contents
Principles of the relevant aspects of Digital Signal Processing 3
Introduction to digital simulation of audio reverberation 4
Acoustic Echo Simulator 5
A Delay Line ... The fundamental building block 6
Audio Discrete Time 7
The Digital Delay Line Length 8
Reverberation is the sum of ... 9
MATLAB® Overview 10
Questions please … 21
Day 1 Task 22
Day 2 Task 23
Credits 24
In addition to the resources found below there are supporting documents which should be used in combination with this resource. Please see: Mini Projects - Introductory presentation. Mini Projects - E-Log. Mini Projects - Staff & Student Guide. Mini Projects - Standard Grading Criteria. Mini Projects - Reflection. You will also need the ‘Mini Projects – Audio Enhancement’ Word document. Mini Project- Audio Enhancement

Principles of the relevant aspects of Digital Signal Processing (DSP)
DSP elements
Basic principles of discrete-time signals
Introduction to digital filtering
Mini Project- Audio Enhancement

Introduction to digital simulation of audio reverberation
The physics of reverberation in closed space environments (refresher)
Introduction to DSP tools for generating artificial reverberation

Acoustic Echo Simulator
Echoes occur when a sound arrives via more than one path
We may hear a discrete echo if we clap our hands standing in front of a large flat wall outdoors. To be perceived as an echo the reflection must arrive well after the direct signal
A mathematical model for an echo.
Mini Project- Audio Enhancement Input Delay Output Delay Mix

A Delay Line ... The fundamental building block
A delay takes an audio signal, and plays it back after the delay time . The delay time can range from several milliseconds to several seconds
This produces a single, delayed copy of the input, and thus is often referred to as an echo device.

Audio Discrete Time
Stereo CD quality is defined at 44,100 samples per second in each channel
There is therefore up to 22,050 kilohertz is available in each stereo channel
Each acoustic amplitude sample is taken at 1/44,100 seconds or 0.02267 milliseconds.
This is referred to as T, the sampling interval

The Digital Delay Line Length
The delay-line is like the fixed ``air'' which propagates sound samples at a fixed speed
c = 344 meters per second at 22 0 C and 1 atmosphere)
If the listening point is d meters away from the source and the sampling period is T, then the delay line length needs to be:
M = d / c T samples
For example if a source is 10 meters away then M needs to be 1282 samples long assuming a CD quality recording.

Reverberation is the sum of ...
Direct sound
From source to ear with no reflections (anechoic)
Early reflections
From a model of the room walls and wall coverings
Late reverberation
From complex interactions decaying over time
Sufficient reflections density to avoid flutter echoes

MATLAB ® Overview
What is MATLAB ® ?
History of MATLAB ®
Who developed MATLAB ®
Why MATLAB ® was developed
Who currently maintains MATLAB ®
Strengths of MATLAB ®
Weaknesses of MATLAB ®

What is MATLAB ® ?
MATLAB ®
MATrix LABoratory
Interactive system
Will be covered during week 2
Programming language
Will be covered during week 3

What is MATLAB ® con’t: 2
Considering MATLAB ® at home
Standard edition
Available for roughly 2 thousand dollars
Student edition
Available for roughly 1 hundred dollars.
Some limitations, such as the allowable size of a matrix

History of MATLAB ®
Ancestral software to MATLAB ® Fortran subroutines for solving linear (LINPACK) and eigenvalue (EISPACK) problems
Developed primarily by Cleve Moler in the 1970’s

Digital Signal Processing for Music
Research in music, as well as music production and composition increasingly relies on sophisticated digital signal processing techniques. This course will review fundamental elements of digital audio signal processing, such as sinusoids, spectra, digital filters, and Fourier analysis and their application to the fundamental music analysis problems of modelling and synthesis. The course will focus particularly on the algorithmic implementation sound transformation and synthesis techniques through intensive programming assignments in Matlab.

History of MATLAB ® , con’t: 2
Later, when teaching courses in mathematics, Moler wanted his students to be able to use LINPACK and EISPACK without requiring knowledge of Fortran
MATLAB ® developed as an interactive system to access LINPACK and EISPACK

Signal Processing Techniques for Digital Audio Effects
Digital signal processing methods for audio effects used in mixing and mastering will be covered. Topics include techniques for dynamic range compression, reverberation and room impulse response measurement, equalization and filtering, and panning and spatialization, with attention given to digital emulation of analog processors and implementation of time varying effects. Among the effects studied will be single-band and multiband compressors, limiters, noise gates, de-essers, feedback delay network and convolutional reverberators, flangers and phasors, parametric and linear-phase equalizers, wah-wah and envelope-following filters, and the Leslie.
The course material will be presented in daily lecture sessions with laboratory exercises interspersed. The lecture sessions will concentrate on theoretical issues in the design of digital audio effects, and are complemented by laboratory work in which students will develop effects algorithms of their own design.
The course is geared for musicians and recording engineers with an engineering background, and for engineers and computer scientists with an interest in music technology. An exposure to digital signal processing, including familiarity with digital filtering and the Fourier Transform is helpful. Some knowledge of Matlab® .

History of MATLAB ® , con’t: 3
MATLAB ® gained popularity primarily through word of mouth because it was not officially distributed
In the 1980’s, MATLAB ® was rewritten in C with more functionality (such as plotting routines)

History of MATLAB ®, con’t: 4
The Mathworks, Inc. was created in 1984
The Mathworks is now responsible for development, sale, and support for MATLAB ® The Mathworks is located in Natick, MA
The Mathworks is an employer that hires co-ops through our co-op program

Strengths of MATLAB ®
MATLAB ® is relatively easy to learn
MATLAB ® code is optimized to be relatively quick when performing matrix operations
MATLAB ® may behave like a calculator or as a programming language
MATLAB ® is interpreted, errors are easier to fix
Although primarily procedural, MATLAB does have some object-oriented elements

Weaknesses of MATLAB
MATLAB ® is NOT a general purpose programming language
MATLAB ® is an interpreted language (making it for the most part slower than a compiled language such as C++)
MATLAB ® is designed for scientific computation and is not suitable for some things (such as parsing text)

Questions please … Mini Project- Audio Enhancement

Day 1 Task
To record a sound (in .wav format) supplied by yourself and design an Infinite Impulse Response (IIR) network to make it sound like it was played in a large cavern with strong echoes.

Day 2 Task
To build and demonstrate a complete set of digital filters connected to process a studio recorded version of the 4th movement of Beethoven Symphony number 9 „Coral“.
The output result should sound like a full concert hall performance.

© University of Hertfordshire 2009 This work is licensed under a Creative Commons Attribution 2.0 License . The name of the University of Hertfordshire, UH and the UH logo are the name and registered marks of the University of Hertfordshire. To the fullest extent permitted by law the University of Hertfordshire reserves all its rights in its name and marks which may not be used except with its written permission. The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that licence. The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher. Credits This resource was created by the University of Hertfordshire and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme. Mini Project- Audio Enhancement

Mini Project- Audio Enhancement

by University of Hertfordshire, School of Electronic Communications and Electrical Engineering

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